We have undertaken to study the mechanism whereby glucocorticoids inhibit proliferation of lymphoma and fibroblast cell lines. Our studies concentrate upon two murine cell lines; the thymic lymphoma line P1798 and a clone isolated from L929 fibroblasts (designated L929.06). These lines respond to dexamethasone (dex) in an identical fashion. Both cease to proliferate in culture in 0.1muM dex. Thymidine kinase activity is reduced by >90% within 24h. Neither cell line exhibits any detectable loss of viability and inhibition of proliferation is completely reversible. Cessation of cell division is accompanied by G1 arrest and is preceded by rapid inhibition of expression of c-myc. This is associated with inhibition of initiation of transcription of the gene. Experiments are proposed to study the mechanism of inhibition of c-myc expression and to test the hypothesis that inhibition of c-myc expression is sufficient to account for inhibition of proliferation of P1798 and L929 cells. To this end, three series of experiments will be carried out. The sequence elements required for glucocorticoid regulation will be defined by DNA- mediated gene transfer experiments using derivatives of the mouse c-myc promoter fused to reporter gene. In parallel, we will characterize those factors required for transcription from the c-myc promoter in vitro and ascertain if any of these is regulated by glucocorticoids. Finally, we will use DNA-mediated gene transfer techniques to determine if glucocorticoid inhibition of proliferation can be circumvented by introduction of a chimaeric gene in which the coding sequence of c-myc is fused to a constitutive or inducible promoter.